Seafloor spreading on the Amsterdam-St. Paul hotspot plateau

The Amsterdam-St. Paul (ASP) platform on the intermediate rate Southeast In dian Ridge (SEIR) is the only oceanic hotspot plateau outside the Atlantic Ocean containing an active, mid-ocean ridge spreading axis. Because the ASP hotspot is small and remotely located, it has been relatively unstudied, a nd the ridge axis location in many places near the ASP plateau was previous ly unknown or ambiguous. We mapped the SEIR out to 1 Ma crust (Jaramillo an omaly) both on and near the ASP platform. We located the spreading center t o within a few kilometers, based on side-scan sonar reflectivity. Recent of f-platform magnetic anomalies and lineated abyssal hill topography are cons istent with a simple spreading history. Off-platform full spreading rates i ncrease from similar to 63 km/Myr on segment H to the north of the platform to similar to 65.5 km/Myr on segment K to the south. In contrast, inversio ns of seafloor magnetization based on uniform and variable thickness magnet ic source layers reflect a complex on-platform tectonic history with ridge jumps, off-axis volcanism, and propagating rifts. On one section of the ASP plateau the spreading location has stabilized and is beginning to rift the plateau apart, generating symmetric magnetic anomalies and lineated topogr aphy for the last several hundred thousand years. The larger, more stable, spreading segments of the ASP platform are aligned with major volcanic edif ices, suggesting that along-axis magma flow away from plume-fed centers is an important influence on spreading geometry. Many complex tectonic feature s observed on the ASP plateau, such as ridge jumps, en echelon, oblique spr eading centers, and transforms oblique to the spreading direction, are comp arable to features observed on Iceland. The similarities suggest that moder ate crustal thickening at an intermediate rate spreading center may have si milar effects to pronounced thickening at a slow rate spreading center.